WebmFrameThread: Don't set mDone in WebmFrameSinkThread::stop

WebmFrameSinkThread::stop shouldn't set mDone to true, it will
be set once EOS frames are handled in WebmFrameSinkThread::run

Bug: 177376580
Bug: 267933226
Test: atest CtsMediaV2TestCases:EncoderProfileLevelTest
Test: atest CtsMediaV2TestCases:MuxerTest
Test: atest CtsMediaV2TestCases:MuxerUnitTest

Change-Id: I0e3bd689b4284104ed8d9e7a84f78598ca413615